The present invention relates to apparatus and methods for cardiac ablation and to sensor structures useful in such apparatus and methods.
Contraction or “beating” of the heart is controlled by electrical impulses generated at nodes within the heart and transmitted along conductive pathways extending within the wall of the heart. Certain diseases of the heart known as cardiac arrhythmias involve abnormal generation or conduction of the electrical impulses. One such arrhythmia is atrial fibrillation or “AF.” Certain cardiac arrhythmias can be treated by deliberately damaging the tissue along a path crossing a route of abnormal conduction, either by surgically cutting the tissue or by applying energy or chemicals to the tissue, so as to form scar. The scar blocks the abnormal conduction. For example, in treatment of AF, it has been proposed to ablate tissue in a partial or complete loop around a pulmonary vein within the vein itself, within the ostium or opening connecting the vein to the heart, or within the wall of the heart surrounding the ostium. It would be desirable to perform such ablation using a catheter-based device which can be advanced into the heart through the patient's circulatory system.
As described in co-pending, commonly assigned U.S. Pat. No. 6,635,054, the disclosure of which is incorporated by reference herein, an expansible structure is used as a reflector for directing and focusing ultrasonic waves from an ultrasonic transducer into a region of tissue to be ablated. As further described in the '054 patent, certain preferred embodiments according to that disclosure include an expansible structure incorporating a structural balloon which is inflated with a liquid and a reflector balloon inflated with a gas. The balloons share a common wall. The balloons are configured so that the common wall has a generally parabolic shape. Because the liquid in the structural balloon and the gas in the reflector balloon have substantially different acoustic impedances, the interface between the balloons at the common wall is a nearly perfect reflector for ultrasonic waves. Ultrasonic waves are emitted from a small transducer within the structural balloon and passes radially outwardly from the emitter to the reflector. The reflector redirects the ultrasonic waves and focuses it into a ring-like ablation region encircling the central axis of the emitter and balloons. This ablation region is just forward of the structural balloon. Thus, the ultrasonic waves will ablate tissue in a region encircling the central axis or forward-to-rearward axis of the balloon structure.
This arrangement can be used, for example, to treat atrial fibrillation by ablating a circular region of myocardial tissue encircling the ostium of a pulmonary vein. The ablated tissue forms a barrier to abnormal electrical impulses which can be transmitted along the pulmonary veins and, thus, isolates the myocardial tissue of the atrium from the abnormal impulses. To provide effective treatment in this mode of operation, the ring-like focal region should encircle the ostium and should lie in a plane which is parallel or nearly parallel with the inner surface of the heart. In some embodiments disclosed in the '054 patent, the structural balloon is provided with a forwardly projecting tip at its central or forward-to-rearward axis.
As disclosed in commonly assigned U.S. Provisional Patent Application Ser. No. 60/448,804, filed Feb. 20, 2003, and in commonly assigned, co-pending U.S. Published Patent Application No. 2004/0176757 (hereinafter “the '757 application”) and PCT International Application No. PCT/US04/05197, the disclosures of which are incorporated by reference herein, a catheter-carried expansible ablation structure as disclosed in the '054 patent can be equipped with a steering mechanism so that the orientation of the expansible structure relative to the heart can be controlled by the physician without relying upon physical engagement with the pulmonary vein or pulmonary vein ostium. This allows the apparatus to be positioned with the loop-like region lying in a plane substantially parallel to the heart wall in the region to be ablated, regardless of the orientation of the pulmonary veins relative to the heart wall and regardless of the exact anatomy of the pulmonary vein ostium in the particular patient to be treated.
As also disclosed in the '757 application, the catheter and the expansible structure desirably define a continuous passageway extending from the proximal end of the catheter to the distal or forward side of the expansible structure, and communicating with a port open on the distal side of the expansible structure. A contrast medium can be injected through this port while the device is in its expanded condition. This allows the physician to obtain an image of the expansible structure along with the heart and pulmonary veins before or during application of ultrasonic energy, so as to confirm proper placement of the device to form a lesion in the desired location. Further, the steering arrangement allows the physician to reposition the device so as to form multiple lesions. The lesions formed may include both loop-like lesions surrounding a pulmonary vein ostium and substantially linear lesions formed by placing the plane of the loop-like ablation region at a substantial angle to the plane of the heart wall, so that ablation occurs only along a small sector of the loop-like ablation region.
It is often desirable to monitor electrical signals propagating within the heart. For example, McGee et al., U.S. Pat. No. 5,860,920, discloses a structure incorporating an elongated element with numerous electrodes disposed along a distal region of the structure. The structure is advanced into the heart within a guide tube or sheath, which is then retracted so as to expose the distal region. In this condition, the distal region, under its own resilience, forms itself into a hoop shape, which can be pressed into engagement with a region of the heart wall as, for example, a region surrounding the bicuspid valve or the mitral valve. The electrodes pick up electrical signals propagating within the heart. The electrodes can be connected to a source of electrical energy, so that the electrical energy applied through the electrodes ablates the cardiac tissue. Swanson et al., U.S. Pat. No. 5,582,609, discloses another loop-forming structure carrying electrodes for electrical ablation. Fuimaono et al., U.S. Pat. No. 6,628,976, discloses a catheter with a similar loop-like structure said to be useful in mapping electrical activity or “wavelets” within a pulmonary vein, coronary sinus or other “tubular structure” prior to treatment of the condition.
Marcus et al., U.S. Pat. No. 5,295,484, discloses a catheter carrying both an ultrasonic transducer and electrodes for sensing electrical potentials within the heart. These electrodes can be used to allow the physician to determine whether the arrhythmia has persisted after the ablation process. Also, the aforementioned '054 patent and '054 patent disclose, in certain embodiments, expansible balloon structures having ring-like electrodes thereon for detecting electrical signals within the heart.
Despite all of these efforts in the art, however, still further improvement would be desirable. Providing electrical sensing structures on a balloon-like or other expansible ablation device complicates fabrication of the device and makes it more difficult to make the device collapse to a small diameter for advancing or withdrawing the device through the vascular system. Further, mounting the electrodes on the same catheter as an ultrasonic transducer, as disclosed in the '484 patent, limits placement of the electrodes and the configuration of the transducer array and associated structures. The particular structures shown in the '484 patent, for example, are not well suited to formation of a ring-like lesion or sensing of electrical potentials at numerous locations. Use of a loop-forming sensing element entirely divorced from an ablation device, as contemplated in U.S. Pat. No. 6,628,976, necessarily requires separate steps for placement of such a device which adds both complexity and risk to the procedure.